The invention relates to a system for admixing a reducing agent to an exhaust gas flow of an internal-combustion engine.
Concerning the technical background, reference is made, for example, to German Published Patent Document DE 40 38 054 A1. In DE 40 38 054 A1, a system is described for admixing a reducing agent to an exhaust gas flow of an internal-combustion engine having an exhaust line and a metering device for feeding the reducing agent into the exhaust line. The system further contains a tripartite evaporator for evaporating the reducing agent, the first part through which the exhaust gas flows consisting of a metallic honeycomb body whose fins can be heated by the exhaust gas. The heating fins of the metallic honeycomb body are arranged to be aligned in the flow direction of the exhaust gas. For admixing the reducing agent, a nozzle device is angled 90° and projects into the exhaust line. By means of this nozzle device, reducing agent is added to the exhaust gas, which reducing agent evaporates on the heated heating fins and thereby becomes chemically active in order to eliminate nitrogen oxides (NOx) in a special nitrogen oxide reduction catalyst. A urea water solution or ammonia is preferably used as the reducing agent.
From the unpublished German patent application having official file number DE 10 2006 049 620.5, on which the present invention is based, a system for admixing a reducing agent, such as a urea water solution or ammonia, to the exhaust flow of an internal-combustion engine, is also known. This extremely advantageous further development distinguishes itself from the above-mentioned state of the art by the fact that the metering device is arranged in or on a hollow body arranged at the exhaust line, the inside volume of the hollow body being connected with the exhaust line in an exhaust-gas-conducting manner. By means of the suggested further development according to the invention, a particularly favorable flow resistance, i.e. a particularly low flow resistance, is achieved and the power of the internal-combustion engine is thereby increased.
However, despite the fact that the further development is extremely favorable with respect to the flow, reducing agent may still deposit on the metering device or on the hollow body, which, in turn, may have a negative effect on the particularly favorable flow resistance.
There is therefore needed a measure by which deposits of reducing agent are decreased or avoided.
This and other needs are met by a system for admixing a reducing agent to an exhaust gas flow of an internal-combustion engine, having an exhaust line and a metering device for feeding the reducing agent into the exhaust line. The metering device is arranged in or on a hollow body arranged on the exhaust line, the inside volume of the hollow body being connected in an exhaust-gas-conducting manner with the exhaust line. The reducing agent is feedable by nozzle into the hollow body. A jacket body is provided between the hollow body and the nozzle-fed reducing agent, which jacket body surrounds the reducing agent at least in areas in the inside volume.
As a result of the further development according to the invention, the combination of a flow-optimized hollow body in connection with the jacket body according to the invention, which surrounds the reducing agent in the inside volume at least in areas, provides a remedy here.
According to a further development, the jacket body can be heated electrically and/or by an exhaust gas. Thus, the deposits of the reducing agent can be evaporated in a simple manner without any problems and can therefore be removed without residue and in a manner that is not toxic to the environment. The heating has the purpose of keeping the jacket body in a desired temperature range. There are basically two heating strategies. At high temperatures above 300° C., the deposits can virtually be burnt off. Otherwise, the heating can be used for adjusting the temperature such that a catalytic coating can have an optimal effect. For this purpose, temperatures of above 200° C. are required. The heating can, for example, take place electrically by way of a heating conductor wound around the jacket body. When the temperature dependence of the electric resistor of the heating conductor is sufficiently high, a temperature control can be carried out without any additional sensor system. However, the jacket body may also be designed such that a heating can be achieved by way of the exhaust gas flowing past it.
According to further developments, the jacket body is catalytically coated at least in areas. The catalytic coating may be titanium dioxide or aluminum oxide. The catalytic coating acts as a hydrolysis catalyst for the reduction decomposition products. Particularly preferably, for example, the titanium dioxide (TiO2) coating or the aluminum oxide (Al2O3) coating is contemplated as the catalytic coating. Catalytic coatings can be effective only if they have a sufficiently large surface and if the required temperature range is observed. The required coating surface is ensured by means of the surface of the jacket body. As a result of the fact that exhaust gas can also flow around the jacket body or the jacket body can be heated electrically, the temperature of the coating can be maintained to be correspondingly high.
The desired thermal behavior of the jacket body is assisted by thermally insulating the jacket body at least in areas. The use of an air gap insulation is a particularly preferred further development for the thermal insulation.
By means of a corresponding shaping-out of the jacket body, the metering device is protected from hot exhaust gas and a so-called flash boiling of the reducing agent by an excessively high temperature at the metering device is prevented.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawing.